1. Field of the Invention
The present invention relates to a process for curing polymerizable liquid compositions of polyisocyanates and epoxies. In particular, the present invention relates to a process for curing liquid compositions containing at least one compound having more than one epoxide group and at least one compound having more than one isocyanate group, in the presence of non-ionizing electromagnetic radiation in the microwave or radio range of frequencies. The material which is produced is a high-performance thermoset which can be quickly formed from a liquid, low-viscosity starting material. The resin hardening, which is preferably followed by a post-curing treatment, must occur quickly at room to moderate temperatures, without the occurrence of overheating phenomena due to the reaction exotherm.
2. Discussion of the background
Industrial sectors calling for both high-performance and rapid curing resins are:
a) Structural fiber-reinforced articles which are fabricated by fast processes such as resin transfer molding, structural reaction injection molding (RIM), pultrusion, and pulforming, for land transportation, boats, aerospace, chemical and petrochemical industries, off-shore drilling, etc.;
b) Insulation and encapsulation of electrical/electronic devices by pressure or vacuum casting;
c) Electrical/mechanical devices insulators, connectors, electrodes, switches, etc.) to be fabricated by RIM or RRIM.
Performance requirements of suitable thermoset resins are high softening point (&gt;200.degree. C.), low flammability, hydrolytic resistance, chemical and solvent resistance, and dielectric rigidity. A broad range of high performance resins are already available: epoxy resins with anhydride or aromatic polyamine curing agents, polyimides and other heterocyclic resins, bismaleimides, etc. These resins are characterized by important drawbacks: i) in general, slow hardening cycles (unsuitable for fast processing) unless high temperatures are adopted, causing in turn, overheating; ii) poor hydrolytic stability and chemical resistance because ester, amide, imide linkages are easily hydrolyzed; iii) high viscosities (epoxy systems generally have viscosities in the range of thousands of centipoise at room temperature).
It is known that polymeric products containing isocyanurate and/or oxazolidone moieties are attainable by polymerization of mixtures of polyisocyanates and epoxy compounds. Thermoset materials prepared from polyfunctional isocyanates and epoxies are characterized by softening temperatures, generally higher than 250.degree. C., excellent hydrolytic stability, chemical and solvent resistance, low flammability (improved up to the self-extinguishing point by addition of mineral fillers), and low dielectric constant (good dielectric properties). Reactive compositions constituted by liquid isocyanates and epoxy resins can be easily prepared with viscosities of a few hundred centipoise at room temperature.
It is known as well, that the polymerization of liquid mixtures of diisocyanates or polyisocyanates and monoepoxides or polyepoxides can be promoted by using tertiary amines, quaternary ammonium salts, or tetra-alkyl phosphonium halides. See, for example, DE 3,323,084, DE 3,323,122, DE 3,323,123, DE 3,323,153, DE 3,600,767; U.S. Pat. No. 3,687,897 and U.S. Pat. No. 4,742,142.
According to these and other patents, the polymerization of mixtures comprising polyisocyanates, polyepoxides and a suitable catalyst is usually accomplished, with subsequent gelation and hardening, by heating at temperatures within the range of from 60.degree. C. to 150.degree. C. and, preferably, within the range of from 80.degree. C. to 130.degree. C. The polymerization is subsequently completed by maintaining the solidified material at temperatures higher than 150.degree. C. for a prolonged period of time, normally a matter of hours.
Although more active catalysts have been described (boron trifluoride and its complexes with a variety of compounds including alcohols, ethers, amines, or amides, etc.), for example, in U.S. Pat. No. 4,705,838 and Japanese patents 57 00 3812, 57 00 3813, and 57 00 3814, they are unsuitable for most industrial applications. Despite the fact that they allow the hardening to occur at room temperature, hydrofluoric acid may be evolved by boron trifluoride hydrolysis in the cured material under the influence of atmospheric moisture. Hydrofluoric acid can be responsible for corrosion of metal inserts, glass fibers, ceramic devices, etc. For this reason, neutral (e.g., quaternary ammonium or phosphonium salts) or slightly basic compounds (tertiary amines) are largely preferred as the isocyanate/epoxy catalysts, despite the fact that their lower catalytic efficiency requires, for hardening in a few minutes or a few tens of minutes at 20.degree.-80.degree. C., catalyst concentrations of up to 3-4%.
Unfortunately, the compositions which are known and therefore the finished articles which can be obtained from them, are not completely free from drawbacks, in particular associated with the catalytic systems used. Among others, some of these deficiencies are:
(a) Fast polymerization rates are only possible at considerably high temperatures, or in the presence of high catalyst concentrations.
(b) The exothermic polymerization reaction produces local overheating in the core of the articles, especially when thick shaped, which can cause decomposition, bubbling and embrittlement of the final polymer.
(c) The external heating of the compositions in order to reach the curing or post-curing temperatures produces local overheating on the surfaces of the articles made, unless a very slow heating process is used.
In order to solve the problem of overheating, cooling systems have been studied. However, these have proven not to be sufficient for very large and/or thick articles. Moreover, cooling can slow down the polymerization after the reaction peak exotherm, therefore requiring very long post-curing cycles.
It is known that some substances, having a high dielectric loss factor, can be heated by submitting them to an electromagnetic field in the microwave range of frequencies. Microwave treatments have been proposed for accelerating the cure of particularly suitable thermosetting resins, where the presence of highly dipolar groups (i.e., sulfonyl or ester) provides sensitivity to microwave radiation. Examples are: (a) curing of an epoxy resin system containing the diglycidylether of bisphenol A and 4,4'-diaminodiphenylsulfone (DDS) (See for example, J. Wei et al, Proc. 5th Amer. Soc. Comp., 1990, page 239), whose sulfonyl group implies a very slow polymerization process, still unsuitably slow for fast hardening, as required by RTM, RIM, etc., even under the influence of microwaves; (b) curing of unsaturated polyester resins, whose final performance is in the range of commodity materials (glass transitions less than 140.degree. C., up to 180.degree. C. with the best vinyl-ester resins, poor hydrolytic and chemical resistance and very poor flame resistance) and are unsuitable for heavy-duty thermal, chemical, and electrical applications.
Despite considerable research in the field, a need continues to exist for improved methods of producing low-viscosity thermoset resins having good thermal, chemical, mechanical and dielectric properties.